Environmental control systems for aircraft typically employ air cycle machines to cool and condition pressurized air supplied by the engines or by an auxiliary power unit. In an air cycle machine, a shaft is rotatably supported in a housing and serves to connect a compressor and a turbine. The pressurized supply air passes first to the compressor where it is compressed and is heated as a result of the compression. After being cooled by a heat exchanger, the compressed air is expanded in the turbine and is chilled to a very low temperature in order to cool the aircraft cabin and the aircraft avionics. The compressed air acting on the turbine rotates the shaft which, in turn, drives the compressor.
To support the shaft which connects the compressor and the turbine, an air cycle machine typically employs three bearings. Two of the bearings are radial bearings which prevent the shaft from shifting radially. The third bearing is a thrust bearing which holds the shaft in a fixed axial position. For optimum performance, very small clearances must be maintained between the machine housing and the tips of the compressor and turbine blades. If the bearings permit more than just slight amounts of free play, the shaft will shift when loaded and will allow the blade tips to contact the encircling housing.
In prior air cycle machines, hydrodynamic fluid film bearings (commonly called air bearings) have been used to locate the shaft radially and axially since such bearings provide minimal free play. Air bearings, however, require extremely small clearances (e.g., .+-.0.0002"), require supply air for cooling, and are slightly damaged at each start up and shut down as a result of lack of support at low speeds. Because of the small clearance in the air bearings, dirt or other combination increases the susceptibility for damage and, in addition, such clearances result in relatively high friction and heat. As a result, bearing replacement is a frequent cost item and, if failure occurs during operation, the compressor and/or turbine may be damaged. A shaft which is supported by air bearings must be removed axially from the housing for repair or replacement and this usually requires that the air cycle machine be removed from the aircraft and sent to a remote repair depot.
Magnetic bearings are used for supporting shafts in various types of machinery. In radial magnetic bearings, several electromagnets are spaced angularly around a shaft and, when energized, produce opposing magnetic forces which cause the shaft to levitate in free space within the housing. Sensors detect the actual position of the shaft and vary the energization of the electromagnets in such a manner as to keep the shaft centered precisely on a predetermined axis.
While the initial cost of a magnetic bearing system may be somewhat higher than that of an air bearing system, magnetic bearings permit more easily attainable machining tolerances (e.g., .+-.0001") and larger clearances, require no air for cooling, experience a relatively long service life, and are capable of supporting shafts either at rest or operating at speeds of 100,000 RPM or higher.